Method for evaporating aluminum

ABSTRACT

Method for evaporating aluminum utilizing a carbon liner having a recess therein by depositing the aluminum in the recess and then heating the aluminum in the recess in a location generally at the center of the recess. The outer margin of the liner is cooled to a greater extent than the inner portion of the liner so that the outer margin of the liner is cooler whereby the outer edge of the aluminum in the recess will be cooled to such an extent that the outer margin is not molten so that the aluminum will not creep over the edge of the liner.

United States Paent [1 1 Sp 1 Dec. 4, 1973 [54] METHOD FOR EVAPORATINGALUMINUM 3,437,734 4/1969 Roman et al. 13/31 3 29 24 7 6 [75] Inventor:Vernon C. Spellman, Santa Rosa, 5 12 1:3

3,029,777 4/1962 Cerych et al 266/34 R [73] Assignee: Optical CoatingLaboratory, Inc.,

Santa Ro c lifl Primary Examiner-Gerald A. Dost Art P lD.Fl h tal. 22Filed: Mar. 13, 1972 omey e r e pp 233,936 57 ABSTRACT Related [15.Application Data Method for evaporating aluminum utilizing a carbon [62]Division of Ser. No. 29,410, April 17, 1970, Pat. No. liner having arecess therein y depositing the alumi- 3,684,263 num in the recess andthen heating the aluminum in the recess in a location generally at thecenter of the [52] U5. C1. 266/34 R, 118/49 reeess- The outer m g of hliner i ool d o a [51] Int. Cl. .L C23c 17/00 greater extent than h nn rpo io of the liner so [58] Field of Search 13/31; 266/34 R, that theouter m gin f h liner is ool r whereby 266/39, 43; 263/48; 118/49, 49,1495 the outer edge of the aluminum in the recess will be cooled to suchan extent that the outer margin is not [56] Referen Cit d molten so thatthe aluminum will not creep over the Smith l3/3l edge of the liner.

2 Claims, 2 Drawing Figures METHOD FOR EVAPORATING ALUMINUM CROSSREFERENCE TO RELATED APPLICATION This application is a divisional ofapplication Ser. No. 29,410, filed Apr. 17, 1970 now US Pat. No.3,684,268.

BACKGROUND OF THE INVENTION In the evaporation of certain materials invacuum chambers certain problems are encountered. For example, in theevaporation of aluminum, difficulty has been encountered in that thealuminum, when melted, wets the surface of the material which isutilized for the crucible or the liner for the crucible so that uponcooling the aluminum contracts and cracks the liner which have beenutilized. There is, therefore, a need for a new and improved sourcewhich can be utilized for evaporating materials.

SUMMARY OF THE INVENTION AND OBJECTS The source for use in evaporatingmaterials consists of a crucible which is formed of a good heatconducting material. The crucible is provided with an upwardly facingrecess which is formed by outwardly and upwardly tapered side walls. Aliner is seated in said recess. The liner is formed of a materialdifferent from the crucible. The liner also has an upwardly facingrecess formed therein. The liner is also provided with an outer sidewall which is tapered upwardly and outwardly so that its taper issubstantially identical to the taper of the side walls forming therecess in the crucible, the size of the liner being such so that asubstantial portion of the tapered outer surface of the liner is incontact with the tapered side wall of the crucible. It is alsopreferable that the liner have a size so that its bottom wall is spacedabout the top surface of the bottom wall for the recess in the crucible.

In general, it is an object of the present invention to provide a sourcefor use in evaporating materials in a vacuum in which the liner will notbreak or crack during heating and cooling of the source.

Another object of the invention is to provide a source of the abovecharacter which is particularly suitable for evaporating metals andparticularly aluminum.

Another object of the invention is to provide a source of the abovecharacter in which the metal will not tend to creep over the edge of theliner.

Another object of the invention is to provide a source of the abovecharacter in which there is a hot molten pool in the center duringoperation and in which the edges of the pool are relatively cool.

Another object of the invention is to provide a source of the abovecharacter in which there is not an undue accummulation of slag duringoperation of the crucible.

Another object of the invention is to provide a source of the abovecharacter which is relatively efficient.

Additional objects and features of the invention will appear from thefollowing description in which the preferred embodiments are set forthin conjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a top plan view of a sourcefor evaporating materials incorporating the present invention.

FIG. 2 is a cross-sectional view taken along the line 22 of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT As shown in the drawings, thesource for the evaporation of materials consists of a crucible 11 whichis formed of a suitable good heat conducting material such as copper.The crucible 11 has a circular geometry. However, it should beappreciated that if desired other geometries can be utilized; forexample, a rectangular geometry. The crucible is provided with arelatively large upwardly facing recess 12. The recess is formed by anupwardly and outwardly tapered side wall 13 and a bottom circular planarbottom wall 14. A downwardly facing recess 16 is also provided withinthe crucible 11 and is adapted to receive a cooling fluid such as waterwhich is utilized for cooling the crucible. The crucible is providedwith a plurality of threaded holes 17 which are adapted to receivescrews so as to make a watertight connection with the base or othersupport member (not shown) which has provisions for supplying thecooling fluid to the recess 16.

A liner 21 is seated within the recess 12 in the crucible. The liner 2-1is formed of a material which is different from the material of thecrucible 11. It is possible that the liner 21 can be formed of differentmaterials and still be useful in the present invention. However, for usein evaporating certain materials such as aluminum, it has been found tobe preferable to utilize a liner which is made of a high purity carbon.The liner is provided with an upwardly facing concave recess 22 whichpreferably is shaped so that it forms part of a sphere, or in otherwords it has a spherical surface. The size of the recess is such that asmall annular planar generally horizontal surface adjoins the side wallforming the recess 22.

The liner 21 is provided with an outer side wall 24 which is alsotapered upwardly and outwardly with a taper which is substantiallyidentical to the taper provided for the side wall 13 of the recess 12 inthe crucible 11. By way of example, the liner may have a thickness ofapproximately five-eighths of an inch at its thickest dimension and maybe provided with a taper of approximately 5. However, if desired, ataper ranging from 2 to can possibly be used.

The liner 21 has a size with respect to the recess 12 and the crucibleso that it will seat within the recess but in such a manner that thebottom surface 26 of the liner is spaced above the bottom wall 14 of therecess 12 to provide a space 27 between the bottom surface 26 and thebottom wall 14 for a purpose hereinafter described. In order to increasethe extent which this space surrounds the liner 21, an annular step orrecess 28 has been provided in the side wall 24 of the liner in such amanner so that it extends upwardly from the bottom surface 26 toapproximately one-half of the side wall. This recess 28 providesadditional air space which is in communication with the space 27.

From the construction noted even with the step or recess 28, there is asubstantial area of direct contact between the upper portion of the sidewall 24 and the side wall 13 forming the recess in the crucible 11.

Operation and use of the source may now be briefly described as follows.Let it be assumed that the source has been provided with a suitablecontinuously flowing cooling fluid and that it is to be utilized inconjunction with appropriate means for supplying heat to the materialwhich is to be evaporated in the crucible. By way of example, one ormore electron guns of a conventional type can be utilized for directingelectron beams into the material in the liner to evaporate the same. Thecrucible can be operated either in a batch mode or a continuous mode. Inthe batch mode, the crucible is preloaded by placing a certain amount ofthe material to be evaporated within the recess 22 of the liner. If thecrucible is to be utilized in the continous mode, wire of the desiredmaterial can be fed continously into the recess of cavity 22. One ormore electron guns are then placed in operation to form a puddle of thematerial.

The electron beams from the guns are positioned so that the hottestpoint in the puddle will be near the center of the puddle and away fromthe outer margins of the liner. The outer margins of the puddle aregenerally cooler than the center of the puddle for the reason that thereis relatively good contact between the upper portion of the liner 21 andthe copper crucible 11 which is water cooled. This means that the outeredge of the liner is continously being cooled and in fact is cooled tosuch an extent that the aluminum will not remain molten at the outeredge of the liner. Since aluminum is not molten at the outer edge of theliner, there is no tendency for the aluminum to move out or creep overthe very edge of the liner but rather will be contained in the centerregion of the liner. The liner 21 which is formed of carbon although itis an excellent conductor also serves as an insulator. Because of thisfact, the temperature of the material being melted in the liner 21increases toward the center. There is also additional insulationprovided between the copper crucible and the liner which insulates thebottom portion of the liner from the crucible. This is the space 27 andthe annular recess 28. This ensures that the hottest portion of thealuminum puddle is not cooled by the water cooled crucible l l but thatonly the upper outer margin of the pool is cooled. In addition, thisgreatly increases the efficiency of the crucible, and makes it possibleto maintain the aluminum very hot so that it will evaporate veryrapidly. The aluminum has a tendency to travel to the hottest spot andtherefore it will have a tendency to travel towards the middle of theliner so that there will be no tendency for the aluminum to creep overthe sides of the liner even though the recess 22 may be very full.

As hereinbefore described, the liner is provided with a sphericalrecess. It has been found that a recess having this shape isparticularly advantageous when the source is used for evaporatingaluminum. It has been found that the aluminum will wet the surface ofthe carbon liner and therefore when the aluminum cools it adheres to thecarbon liner. However, upon cooling it is found that the carbon readilyshears along the interface between the carbon and the cooled aluminumthus preventing the liner from cracking or breaking and thus enablingthe source to be retired without the necessity of replacing the liner.The shearing action takes place because the carbon is considerablyweaker than aluminum. The carbon particles which have been sheared offof the carbon liner up reheating will float out to the outer edge of thepuddle and will form a sort of slag at the extreme edge of the aluminumpuddle along with other impurities that are in the parent metal. Whensufficient impurities and other slag accumulates, it may be necessary toeventually replace the liner. However, it has been found that a liner ofthis type can be reheated or refired many times for the evaporation ofaluminum without the necessity of replacing the same.

The taper which has been provided on the liner and in the recess ensuresthat there is continuously good contact between the upper portion of theliner and the crucible regardless of expansion and contraction duringall stages of heating the liner and the material contained therein. Aspointed out above, this tight interface is necessary to maintain theouter margin of the liner relatively cool so that the aluminum will haveno tendency to creep up the sides of the liner.

It has been found that it is possible to utilize the source for othermaterials as, for example, dielectrics. When such is the case, the linercan be machined so that the recess 22 is much larger and so that thewall thickness of the liner is only approximately .100 inch thick. Inthis way it is possible to greatly increase the capacity of the liner.This type of crucible also operates very satisfactorily because theliner has a slight amount of insulation heat wise as hereinbeforepointed out even though it is a good conductor electrically. Thisgreatly increases the efficiency of the source since large quantities ofthe heat will not be lost through the watercooled crucible.

It is apparent from the foregoing that there has been provided a sourcewhich is particularly useful in evaporating materials in a vacuumchamber and particularly for use with electron guns. There is notendency for the material which is being evaporated to creep over theedges of the liner. In addition, the material does not crack or breakthe liner when it cools. It is particularly satisfactory for evaporatingaluminum which is one of the more difficult materials to evaporate.

I claim:

1. In a method for evaporating aluminum in a carbon liner having anupwardly facing recess therein forming a minor portion of the surface ofa sphere, depositing aluminum in the recess, heating the aluminum in therecess in a location generally at the center of the recess to form acentrally disposed molten puddle of aluminum in the recess, cooling theouter margin of the liner to a greater extent than the inner portions ofthe liner so that the outer margin of the liner is cooler whereby theouter edge of the puddle of aluminum within the recess in the liner willbe cooled to such an extent that the outer margin of the aluminiumwithin the recess is not molten so that the aluminum will not creep overthe edge of the liner.

2. A method as in claim 1 together with the step providing insulationfor the bottom of the liner to minimize cooling of the central portionof the lines.

2. A method as in claim 1 together with the step providing insulationfor the bottom of the liner to minimize cooling of the central portionof the lines.